The present invention relates to a video camera and, more particularly, to apparatus and method for clamping an image signal for use in a video camera.
As is known, analog image signals output from image pickup devices are supplied to a feedback clamp circuit prior to being processed in various manners including, for example, linear mask processing, gamma correction, and knee correction, in order to produce a video signal. The feedback clamp circuit converts the analog image signal to a digital signal, generates an error signal from the difference between the digital signal and a digital-code signal representing a reference level, converts by means of integration the error signal to an analog signal, and adds the analog signal to the originally supplied image signal. The feedback clamp circuit operates to control the level of the analog image signal to a predetermined level. However, to provide linear feedback clamping with a high degree of accuracy, the generated error signal must have a word length of, for example, eight to twelve bits, thus requiring a relatively large number of input pins (i.e., terminals) to exist in the feedback clamping circuit. To overcome this problem, a digitally converted image data signal is compared to a reference level to produce a one-bit flag signal that indicates whether or not the digital image data signal exceeds the reference level, and the flag signal is utilized to determine whether linear feedback clamp processing is necessary. However, since the least significant bit (LSB) of the digital image data signal fluctuates in accordance with the value of the flag signal, the level of the image data signal also fluctuates (i.e., oscillates).
When the image data signal is suppressed by suppressing the level variations thereof (which occur due to the flag signal) to converge the image data signal, such convergence generally takes a relatively long time. Thus, the initial power-on procedure also requires a relatively long time. Still further, convergence of the image data signal also takes a relatively long time when the level of the image data signal exceeds the reference level during an image pickup operation, thus resulting in excessive delays before an image can be picked up by the imaging device.